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Trilobites are among the most popular of fossil groups, and during the Ordovician Period, they often dominated the fossil faunas of mudstone beds deposited on the outer continental shelf. This apparent abundance may be misleading, however: because trilobites have a mineral (calcite) skeleton, they preserved easily, biasing the fossil record in their favour. It is much rarer to discover more delicate organisms in fine detail, although offshore muddy habitats today are often dominated by sponges, brittle-stars and sea-cucumbers instead of arthropods. This paper describes a new deposit from the Middle Ordovician (approximately 462 million years ago) of Wales that has yielded not only the trilobites, but a diverse community of organisms including sponges, palaeoscolecidan worms and the extinct carpoid echinoderms. It also preserves the oldest known complete sea-cucumbers, the bodies of which are so soft that they are very rarely fossilized even in sites of exceptional preservation. These small, globular fossils show primitive features of the group that help relate them to other types of echinoderm. Most importantly, the fauna of this site shows that fossil communities that appear to be typical of Ordovician muddy seas can in reality be ecologically very different, and an analysis of the available data suggests that trilobites were a relatively minor component of the fauna, after brachiopods, sponges and armoured worms.

This email address is being protected from spambots. You need JavaScript enabled to view it.State Key Laboratory of Palaeobiology and StratigraphyNanjing Institute of Geology and PalaeontologyChinese Academy of ScienceNo. 39 East Beijing RoadNanjing 210008China

Joseph Botting is a Young International Scientist at the Nanjing Institute of Geology and Palaeontology. He started working in the Builth Inlier as an undergraduate at Cambridge, and completed his PhD, which was partly on faunas from the area, at the University of Birmingham in 2000. Most of his publications have been on sponges, but he has also worked on echinoderms, worms, trilobites, palaeoecology and several Ordovician Lagerstätten. His research is currently focused on the early evolution of sponges, with forays into exceptionally preserved biotas in China, Morocco, the UK and North America.

This email address is being protected from spambots. You need JavaScript enabled to view it.State Key Laboratory of Palaeobiology and StratigraphyNanjing Institute of Geology and PalaeontologyChinese Academy of ScienceNo. 39 East Beijing RoadNanjing 210008China

Lucy Muir is primarily a graptolite worker and palaeoecologist, but has published on a variety of other things, including museum collections and modern ecology. She received her undergraduate degree from the University of Cambridge (1997) and then undertook an MSc in Palaeobiology at the University of Bristol (1999). Since gaining her PhD from the University of Edinburgh in 2004, she has worked as a museum curator and a freelance geological consultant. She is currently a postdoctoral researcher at the Nanjing Institute of Geology and Palaeontology, where she is working on a comparison of Ordovician Lagerstätten in Wales, China and Morocco.

TABLE 1. Complete list of the fauna known from the Holothurian Bed at the time of writing; a total of ten sponge species and two dendroids are included, although the exact numbers may increase on further study. Abundance notes are semi-quantitative: "rare": encountered on three or fewer occasions; "occasional": encountered on fewer than half of collecting trips; "frequent": encountered in low numbers on most collecting trips; "common": encountered on almost all collecting trips, often with several individuals; "superabundant": encountered on almost every slab.

Group

Taxon

Notes

Trilobites

Ogyginus corndensis

Common, including as complete juveniles

Ogygiacarella debuchii

Moderately common

Cnemidopyge parva

Occasional complete moults

Protolloydolithus reticulatus

Occasional

Platycalymene simulata

Rare – one complete meraspis

Rorringtonia kennedyi

Occasional

Geragnostus mccoyii

Occasional

odontopleurid indet.

Rare – one rib fragment

Microparia lusca

Rare – few near-complete adults

Other cyclopygids (indet.)

Rare fragments

Barrandia cordai

Rare

Brachiopods

Apatobolus? micula

Superabundant

Palaeoglossa attentuata

Common

Monobolina ramsayi

Occasional

Opsiconidion nudum

Occasional

Schizocrania cf. salopiensis

Occasional–only encrusting undetermined substrate

Meristopacha granulata?

Rare

Dalmanellid indet.

Rare

Graptolites

Cryptograptus tricornis

Frequent

Hustedograptus? teretiusculus

Occasional

Dendrograptus spp.

Rare; at least two species

Echinoderms

Cystoid indet.

Rare – one specimen and one possible

Anatifopsis? sp.

Common

Galliaecystis? sp.

Rare – fragments of four specimens

Solute undet.

Rare – one articulated specimen

Carpoid indet.

Rare – one specimen

Oesolcucumaria eostre gen. et sp. nov.

Occasional

Cnidarians

Sphenothallus sp.

rare – few specimens

Conulariid indet.

Rare – one specimen

Ostracodes

Undetermined smooth form

Common

Bullaeferum sp.

Occasional

Nanopsis sp.

Occasional

Chordates

Conodonts

Occasional as isolated elements; rare as clusters (pellets?)

Molluscs

Bivalve indet

Rare

Gastropod indet.

Rare – few specimens

Nautiloids indet.

Rare – few specimens

Sponges

Pirania? sp.

Rare

Wapkia-like form

Occasional

Xylochos? sp.

Occasional

Reticulosan sp. 1

Frequent

Reticulosan sp. 2

Frequent

Hexactinellid indet.

Rare

Sponges undet. (several spp.)

Rare

Eiffeliid indet.

Isolated spicules only

Worms

Palaeoscolecid gen. et sp. nov. (Botting et al. in press)

One confirmed specimen; many too poorly preserved to identify

Palaeoscolecid undet.

One specimen with different plating to Palaeoscolecid gen. et sp. nov.

Scolecodonts

One associated pair of elements

Undet.

Algal remains?

Remains on which ostracodes are feeding

TABLE 2. Approximate proportions of the fauna in terms of collected and life abundance, based on largely non-analytical collecting over 10 years; the figures are therefore not rigorous, but instead estimated values from our observations. The correction factor is based on the ratio of articulated to fragmented scleritomes of Anatifopsis?, representing the proportion of individuals affected by rapid burial, which was then applied to other multi-element scleritomes. Deviations from this factor were employed to allow for ecdysis and additional requirements of soft-tissue preservation; see text for more details.

complete (c)

disarticulated

(d)

Correctionfactor (Ci)

Estimated relativelife abundance = c(Ci+1)

Apatobolus? micula

>100

Many

>>1000

sponges

50

Not visible

20

1050

palaeoscolecids

17

2 fragments

~20

357

trilobites

10 (non-moult)

50 (moults)

?

20

20/5=4

<210

<250

ostracodes

30

Many

20/4=5

180

holothurians

8

Not visible

20+

>168

mitrates

7

~140

20

~147

other brachiopods

?

100

-

100

graptolites

80

n/a

-

80

solutes

1

Not visible

20

21

bivalves

5

n/a

-

5

cornutes

0

4

-

4

nautiloids

3

n/a

-

3

sphenothallids

3

n/a

-

3

gastropods

2

n/a

-

2

TABLE 3. Expected taxonomic composition of the preserved fauna under different degrees of preservation. "Poor preservation" implies an entirely disarticulated and washed assemblage of shelly fragments. "Moderate preservation" implies a fauna lacking abrupt burial and soft tissue, but without extended sea-floor exposure or winnowing. The observed fauna is extensive, but will inevitably be increased by further collecting, as taxa from the tail of the hollow curve are discovered. The total diversity of macrobiota in the local area at the time of deposition is impossible to ascertain, but it was likely to have been in excess of 100 species, by comparison with major Lagerstätten such as the Fezouata Biota (Van Roy et al., 2010) and the Burgess Shale (Conway Morris, 1986). In these deposits, many taxa are localised, and were found only after extensive collecting.

FIGURE 1. Stratigraphic column and map showing the location of the Holothurian Bed locality near Llandrindod, Powys, Wales (inset map shows location within southern UK). Llanfawr Mudstones Lagerstätte refers to Botting et al. (2011), and the Llandegley Rocks Lagerstätte was described by Botting (2005). CMF: Camnant Mudstones Formation. The boundary between the Didymograptus murchisonii and Hustedograptus? teretiusculus biozones is not well constrained, except in the stream itself, where it is somewhat faulted (Sheldon 1987b).

FIGURE 2. Outcrop of the Holothurian Bed, in the upper part of the section; the interval described here overlies a thick tuff bed visible in the upper half of the view. Beds underlying the tuff are blue-grey mudstones with similar fauna to the non-exceptional components found above it. The tuff bed is 27 cm thick.

FIGURE 7. Species-abundance curve obtained from in situ sampling exercise over two days, excluding Apatobolus? micula. Note that the curve appears to level out at approximately 48 specimens, but we are aware of a minimum diversity of 52 species at the site, and the true diversity will be substantially higher.